Scenarios. They are the driving force of your campaign. Heck, they don't even have to be part of a campaign; perhaps it's a one-shot adventure. Either way, you ain't gonna get much roleplaying done without a scenario, and for a scenario, you need plots.

There have been numerious articles done on ways to improvise a scenario on the fly (see The Lazy's man guide to constructing a CoC or Horror adventure and Rock and Role-Playing for examples), but chances are that you don't want to improvise what might be the only adventure in your campaign if you do, unless absolutely necessary. Enter this article.

My purpose in writing this article is to give GMs (mostly budding ones like myself) ways to develop plots/scenarios that can be used in their campaign, or as stand-alone one-shot sessions.

The first piece of advice I can give you is the most basic rule of all GMs: look for inspiration everywhere! I cannot stress enough that you will find plots and story ideas wherever you look, if only you devote a little bit of effort to it. The two most obvious places to look are the movies and books, but unless they're relatively unknown, I'd steer clear of that path. Remember, we're looking to develop our own plots with just a smidgen of help inspiration-wise, not pull plots entirely from another source (otherwise you'd have already done so and wouldn't be reading this, aye?).

Starting Out*
The Basics
Before you start, you must inevitably choose what your scenario is going to focus around. What major event has happened or will happen that sets this scenario in motion? What princess was kidnapped, who kidnapped her, why did they do it, and how is this situation going to be resolved? Obviously that is just the bare minimum of what you'll need, but those are the meat and potatoes of any scenario. Who, what, where, when, and why about "The Event". If you don't have those, chances are the game is going to go roughly as things won't quite fit together.

Okay, so you've now got your basics. The princess was taken by a group of cultists because she is needed for a sacrifice to summon the Supreme Overlord of Darkness into this realm. The first question you should get when looking at that is, "Why do the cultists need the princess for this ritual?" The ritual obviously has some special requirements, so "What exactly does this ritual require?" is the next question to ask. While your players might never see or know these requirements, they will help you greatly in setting up the "final confrontation" scene, as well as help figure out what little snippets of information to give the players so they can figure out how to solve the problem. In fact, those very requirements might be the only way the players get to the final confrontation.

For example, say that the above-mentioned ritual requires that a royal virgin be sacrificed under the light of the Blood Moon within the Unholy Circle of Blasphemy. You now have the basic setting for the final confrontation: it will be at the Unholy Circle of Blasphemy during the Blood Moon, and the characters must prevent the princess from being sacrificed. What exactly that entails (where the Circle is, when the moon will be a Blood Moon, etc.) is mostly up to you, but you'll typically want to handle major descriptions before the session, unless you're a good improv storyteller. Remember, a bit of preparation beforehand is worth a whole heck of a lot of work on the spot.

The Hook
Ah yes, the hook. This is what will snag your players and bring them (and their characters) in. You absolutely positively MUST have at least one plot hook to get the characters into your scenario. A typical stereotype is the classic "You meet up in a bar." hook. While it has its uses, that hook is about as overused as a drow assassin. Look here for advice on how to make better hooks; ephe gave it a better job than I ever could.

These are going to be the heart of the game, obviously. You'll want to handle them immediately after figuring out your major Event. NPCs are the driving force of the game. Without townspeople, cultists, helpless royal virgins and the like, your scenario will be both lifeless and boring. We definitely don't want that to happen, so make sure your NPCs work well with their environment. If the PCs are in a village, make sure there are NPCs to fill every niche available: bartender, drunkard (or five), a "gang" for larger towns, healers/herbalists, etc. Make the people fit, and make the characters interact with them. It promotes that sense of realism that we all want in our games.

Side Plots
While you can run a scenario with just one major event that has to be handled, that often makes things far too linear and one dimensional. My suggestion is that you have at least one Side Plot that the characters can interact with, if not more. This will make things much if you will. It immerses the characters in the environment much more than a singular goal of saving the princess. Obviously, they should be working towards that main goal, but if they have to clear their names of a crime they didn't do at the same time, it makes things much more interesting and fun.

Your NPCs will be vital in making this work. If they players tick off one of the town drunkards, he might not do anything, but he also might go blab to the local gang, which then would pick a fight and generally cause trouble for the PCs.

Making a Side Plot is basically the same as how you start off a scenario: find an event that happens, and build off of it. Heck, a side plot could even have minimal involvement from the PCs. Perhaps the players are merely witnesses to something every day and common, such as a couple having an arguement one day, then the next time the players see them they've made up, and perhaps the PCs hear a few days later of a marriage between the two. Make things real. There ARE other things going on, no matter where the characters go. This sort of thing adds essential realism.

*While this article is mainly about scenarios, I'm quite sure you can use it to develop your campaigns as well. Just broaden the scope a bit.